Offshore seabed to surface conduit transfer system

ABSTRACT

The present invention relates to a transfer system for transferring hydrocarbons, power or electrical/optical signals as the case may be from the seabed to the vessel or other buoyant structure in the shallow water when exposed to the environmental loadings from wind, wave and current. The conduit transfer system comprising a flexible pipe or umbilical extends from the buoyant unit at one end and to the seabed at the other end; and a riser support fixed to the seabed for supporting the flexible pipe characterized in that the flexible pipe a plurality of buoyancy beads for creating one or more inverse catenary curves of the flexible pipe to provide an excursion envelope.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a conduit system for transferringhydrocarbons, power or electrical/optical signals from the seabed to afloating structure in the shallow water when exposed to theenvironmental loadings from wind, wave and current.

BACKGROUND OF THE INVENTION

In offshore oil and gas production a variety of buoyant structures areused to support oil and gas processing equipment, storage facilities orother facilities that condition fluids which are being transferredbetween the seabed and the buoyant structure. Such buoyant structuresincludes monohull vessels type structures known as FSO (Floating StorageOffloading) or FPSO's (Floating Production Storage Offloading) , semisubmersibles, tension leg platforms, caisson structures including spartypes, CALM (Crude and a loading buoy) buoys etc. Common for all thesestructures is that when exposed to a combination of wind, wave andcurrent the structures will displace in the horizontal and verticalplane. Conduits between such structures and the seabed will thereforeneed to incorporate some means of accommodating this relative movement.

Flexible pipes constructed from a combination of rubber, plastics andmetallic elements are frequently used in applications to overcome theabove aforementioned relative displacements. Such pipe structures arecommonly known as “Flexible Pipe” and when applied for the purpose oftransferring flowing media between the seabed and the sea surface thepipe structure is referred to as a “Flexible Riser”.

Umbilicals constructed from small bore pipes and/or electrical cablesand/or optical cables together with possible armor wiring and externalthermoplastic sheeting are frequently used to transfer electrical orhydraulic power and/or electrical or optical control signals between theseabed and the sea surface. The umbilical used in such a dynamicapplication is commonly referred to as a “Dynamic Umbilical”. In thedescription of the invention the focus will be on its applicability toFlexible Riser, however, the invention equally apply to DynamicUmbilicals.

A number of configurations are used when suspending the flexible pipebetween the seabed and the buoyant unit. The main riser configurationsare those known by names of “Free Hanging Catenary”, “Chinese Lantern”,“Steep S”, “Steep Wave”, “Lazy S”, “Lazy Wave” and “Pliant Wave”. Theshape of the various riser configurations is attained by use ofconcentrated or distributed buoyancy elements fixed to the riser andsometimes tethered back to the seabed as well as weight elements andtether lines to seabed anchors.

In all these configurations the flexible pipe is continues from the unitto the seabed except for any joints at some mid water depth required tojoin flexible pipes of different construction is so required or joiningmade via shot length of hard pipe over support structures such as midwater arches employed in for instance the “Steep S” and “Lazy S”configurations.

In particular shallow water and harsh environment a large proportion ofthe total riser length is exposed to hydrodynamic loads from the waveaction and any buoy structures used in producing the riser configurationis equally exposed to said wave loading. This in combination with theexcursions of the surface unit can lead to extreme dynamic movements ofthe flexible pipe with over stressing, over bending of the pipe orunacceptable compressive loads in the flexible pipe. Also buoyantsupport structures may become unstable and exhibit unacceptable dynamicmotions rendering the system unfeasible.

A further complication may arise in shallow water applications when theriser system is used in combination with a surface unit that may attimes be over the riser system, such as for instance with a turretmoored monohull vessel that will weather vane in response to theprevailing environmental loadings. In this instance the available heightfrom the seabed that can be used to generate a riser configuration islimited by the requirement to provide clearance to the bottom of thevessel above, which will experience vertical motions up and down inaddition to the water currents.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the response of theflexible pipe such that a flexible pipe based system can be used inharsher and shallower water applications.

Unlike the known riser configurations where the riser is terminated atthe seabed in close proximity to the mudline in the current inventionthe riser is suspended from a rigid structure extend up from the seabedto somewhere at mid water depth. The connection at this point is fixedand will not be subject to any significant movement even when exposed toextreme environmental loadings. The flexible pipe in its simplestconfiguration is suspended in a catenary configuration between the midwater connection point and the surface unit. This arrangement wasproposed by Kelm et al refer U.S. Pat. No. 7,040,841 B1. However, in thecurrent invention the further buoyancy beads are added to the flexiblepipe to provide one or more inverse catenary shapes along the length ofthe riser, which will further greatly enhance the relative movementsthat can be accommodated between the surface unit and the mid waterfixity points. Buoyancy beads may also be attached to the flexible pipeto add weight locally to change the shape of the configuration and theriser's dynamic response if required. Subject to the seabed conditionsthe riser may be allowed to touch the seabed and external wear modulesmay be attached to the flexible pipe in the touch down area should thisfeature be required.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be clearfrom reading the description hereafter, given by way of non-limitativeexample, with reference to the accompanying drawing wherein:

FIG. 1 illustrates a conduit system of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

Other features and advantages will become more apparent form thedescription of one preferred embodiment of the invention and from theappended drawing depicting the invention.

The present invention relates to a conduit system to carry liquid or gasor a combination of such media i.e. hydrocarbons and or power and orelectrical/optical signals to or from a buoyant unit 1 at or near thesea surface and the seabed 2. In case of hydrocarbons the flow iscarried via a flexible pipe 3 attached to the buoyant unit 1 via aflanged or similar type pipe connection. The flexible pipe is attachedto a fixed point 4 elevated substantially above the seabed mudline 5.The conduit for the media from the fix point 4 to the seabed connectionpoint 6 is via fixed or flexible pipe 7 as the case may be.

The fixed or flexible pipe 7 is supported of a riser support 8preferably a rigid structure that can be constructed from steel,concrete or other rigid construction material. The riser support 8 isfixed to seabed and the structures foundation can be drilled or drivenpiles 9 connected to the structure such suction cans 10 or gravity typefoundation or a combination of such foundations.

The fixed or flexible pipe 7 may include valve arrangement 11 forisolation and manifolding if more than one riser is used, pig launcheror pig receiver. Since such devices can be installed at mid water depthit may allow diver intervention using air diving technique rather thansaturation diving required at greater depth.

The flexible pipe 3 may be fitted with weight modules 12 to change thecatenary shape and/or improve the dynamic response of the flexible pipewhen subject to environmental loadings and movements of the surface unit1. The flexible pipe 3 may be allowed to touch the seabed and can ifrequired be fitted with external wear protection 13.

To accommodate even larger excursions of the buoyant unit 1 the flexiblepipe will be fitted with buoyancy beads 14 to create one or more inversecatenary shapes along the length of the flexible pipe. These inversecatenary shapes greatly enhance the excursion envelope of the surfaceunit the system can accommodate as compared to a riser hanging in asimple catenary shape as proposed in prior art.

The flexible pipe 3 may require bend stiffener 15 to provide a gradualincrease in bend stiffness and so as to not over bend or kink the pipeat or close to its attachment points. The pipe may also be supported byan arch or a gutter structure 16 to avoid the flexible pipe being overbend in the in and out of plane directions. The tether 17 may berequired to anchor and prevent the flexible pipe 3 from over float.

One advantage of the present invention is that it increases theexcursions and movements that the flexible pipe can accommodate with outexceeding its allowable mechanical properties for stress and bending.This reduces the requirement to the mooring arrangement required for thesurface unit and also allows the system to be deployed in more shallowand harsh environment.

Another advantage with the current invention is that it reduces thelength of flexible pipe to be used and since flexible pipe is moreexpensive than normal rigid pipe the invention offers savings onflexible pipe material, which can be significant particular when a largenumber of risers are configured as proposed.

The proposed invention is also less sensitive to changes in the densityof the fluid being conveyed inside the flexible pipe and so can forinstance readily accommodate slug flow where the flow regime consist ofgas flow separated by liquid slugs flowing through the system

1. A conduit system for transferring hydrocarbons from a seabed to afloating vessel or structure comprising: a flexible pipe extends fromthe floating vessel or structure at one end and to the seabed at theother end; and a riser support fixed to the seabed for supporting theflexible pipe characterized in that the flexible pipe is fitted with aplurality of buoyancy beads for creating one or more inverse catenarycurves of the flexible pipe to provide an excursion envelope.
 2. Theconduit system as claimed in claim 1, wherein the flexible pipe isfitted with a plurality of weight modules to regulate a catenary shapeof the dynamic response of the flexible pipe.
 3. The conduit system asclaimed in claim 1, wherein the flexible pipe is attached to a fixedpoint elevated substantially above the seabed.
 4. conduit system asclaimed in claim 1, wherein the riser support is a rigid structure fixedto the seabed.
 5. The conduit system as claimed in claim 1, wherein theflexible pipe is fitted with external wear protection for allowing theflexible pipe to touch the seabed without damaging the flexible pipe. 6.The conduit system as claimed in claim 1, wherein the flexible pipe isprovided with a bend stiffener for allowing the flexible pipe to bendgradually.
 7. The conduit system as claimed in claim 1, wherein theflexible pipe is provided with a tether for anchoring and preventing theflexible pipe from being over floated.
 8. The conduit system as claimedin claim 1, further comprises, a communication system for carryingcommunication signals from a seabed to a floating vessel or structure.9. The conduit system as claimed in claim 1, further comprises a powersystem for power transmission from a seabed to a floating vessel orstructure. 10-16. (canceled)